Various attempts have been made to design seats that support an ergonomic sitting posture, particularly in respect of office chairs and seats for PC workstations. Accordingly, there are designs that resemble rocking-chairs, seats with knee supports and seat-balls. However, all these seats give rise to relatively tense sitting postures, since on the one hand the moving seating surface must be stabilized, and on the other, parts of the body such as the shins, cruciate ligaments and the extremities of the foot are placed under unaccustomed stress.